1. Field of the Invention
Network analyzers are used to measure the scattering parameters of electronic circuits and components in the RF and microwave range (for example network analyzer ZPV-Z5 of Rohde & Schwarz). A network analyzer comprises two test ports between which any desired two-port circuits of a multi-port test item can respectively be linked. It is possible through at least one of these test ports to feed an RF signal from an RF generator into the test item, and measurements by magnitude and phase may then be performed on the two-port of the test item by means of signal detectors such as voltmeters which may be coupled to these test ports via bridge circuits or directional couplers. For the complete determination of the scatter matrix ##EQU1## of a two-port, four complex measurements by magnitude and phase are required at each frequency point, i.e. reflection measurements at the input and the output of the test item (S.sub.11 and S.sub.22) and transmission measurements in both the forward and reverse directions (S.sub.21 and S.sub.12). Since a linear two-port is completely represented by this set of four complex scattering parameters at each frequency point, all further interesting parameters such as input and output impedance, transmission or return loss as well as delays can easily be determined from these scattering parameters by conversion. A network analyzer therefore proves to be a general-purpose and versatile measuring instrument for use at RF frequencies. In practice, the measurements involve a number of errors of measurement brought about by the imperfections of the network analyzer, for example by a finite directivity or mismatch of measuring bridges or directional couplers at the test ports. But these system errors can be fully determined through a calibration process and can subsequently be eliminated by computation. This leads to an improved measurement accuracy and a broader bandwidth of such a network analyzer.
2. Description of the Prior Art
It is known that the imperfections of a network analyzer can be described by twelve complex error measurements on four calibration standards (News from Rohde & Schwarz, 108, winter of 1985/1, pp. 26/27). Furthermore it is also known for such calibration measurements to use only three calibration standards and to determine with a reduced number of measurements all of the error parameters of a network analyzer (HP 8510 TRL calibration technique of Hewlett Packard Product Note 8510-8, October 1987). All of the scattering parameters of the first calibration standard are known, in the simplest case this calibration two-port is realized by a through-connection of the two test ports (Thru measurement). The second calibration standard is a one-port with a high reflection coefficient which is linked to the two test ports in succession (Reflect measurement). The third calibration standard is a short electrical line of random length which is non-reflectively matched to the test ports (Line measurement). The sequence of the various measurements of scatter parameters on these three calibration standards is arbitrary, so that the line measurement could also be performed before the reflect measurement. Although this known calibration technique requires only three calibration standards, the technical implementation, especially of the third calibration standard in the form of the non-reflectively matched electrical line, is relatively difficult. The electrical length of the line has to be chosen so that a certain phase displacement is obtained which is not in the vicinity of 0.degree. and 180.degree., respectively. Using such a line, however, means that with increasing frequency, ranges are repeatedly passed in which the electrical length is in the vicinity of multiples of one-half of the wavelength so that such a line becomes useless for calibration. Moreover, a line always has a lower frequency limit so that it is not a broad-band device, and with increasing electrical length of the line, the relative bandwidth where such a line may be used as calibration standard becomes less and less. Therefore the technical realization of such a coaxial calibration line is relatively difficult.